Hydraulic machines such as dozers, loaders, excavators, backhoes, motor graders, and other types of heavy equipment use one or more hydraulic actuators to accomplish a variety of tasks. These actuators are fluidly connected to a pump of the machine that provides pressurized fluid to chambers within the actuators, and also connected to a sump of the machine that receives low-pressure fluid discharged from the chambers of the actuators. As the fluid moves through the chambers, the pressure of the fluid acts on hydraulic surfaces of the chambers to affect movement of the actuators. A flow rate of fluid through the actuators corresponds to a velocity of the actuators, while a pressure differential across the actuators corresponds to a force of the actuators.
Control over the speed and/or force of hydraulic actuators can be provided by way of one or more metering valves. For example, a first metering valve controls fluid flow into a head-end of a hydraulic cylinder, while a second metering valve controls fluid flow out of the head-end. Likewise, a third metering valve controls fluid flow into a rod-end of the hydraulic cylinder, while a fourth metering valve controls fluid flow out of the rod-end. The different metering valves are cooperatively opened and closed (e.g., based on operator input) to cause fluid to flow into one end of the hydraulic cylinder and simultaneously out of an opposing end, thereby extending or retracting the hydraulic cylinder.
A conventional metering valve includes a body having a bore that receives a spool, and two or more passages formed in the body that communicate with each other via the spool. The spool is generally cylindrical, and includes lands that extend outward away from the body on either side of a valley. When the lands are positioned at one or more entrances of the passages, the spool is in a flow-blocking position. When the spool is moved to a flow-passing position, the valley extends over the entrances such that fluid communication between the passages is established via the valley.
Although conventional spools are acceptable in many applications, they can be massive and require a significant amount of energy to move them between the flow-blocking and flow-passing positions. In addition, because of their mass, the movements of the spools can be slow, causing the associated hydraulic system to be less responsive than desired. The lack of responsiveness caused by the spools may require the use of additional hydraulic components (e.g., mechanical and/or hydro-mechanical compensators) to offset the effects of the slow spools.
One attempt to improve hydraulic system responsiveness is disclosed in a technical paper titled “FLOW FORCES ANALYSIS OF AN OPEN CENTER HYDRAULIC DIRECTIONAL CONTROL VALVE SLIDING SPOOL” written by R. Amirante et al. that published in the Energy Conversion and Management journal in 2006 (“the technical paper”). In particular, the technical paper discloses a hollow spool disposed in the bore of a valve body. The valve body defines a tank port, a pump port, a first work port, and a second work port all in communication with the bore. The hollow spool includes four patterns of radial orifices, wherein two of the patterns are located at a first end of the hollow spool and associated with the first work port, and two of the patterns are located at a second end and associated with the second work port. The two ends of the hollow spool are internally isolated by a block, such that the two ends do not fluidly communicate with each other. The hollow spool is moved between on- and off-positions. In a first on-position, the radial orifices in the first end of the spool connect the first work port with the pump port via the hollow interior of the spool, while the radial orifices in the second end of the spool connect the second work port with the tank port via the hollow interior of the spool. In a second on-position, the radial orifices in the second end of the spool connect the second work port with pump port via the hollow interior of the spool, while the radial orifices in the first end of the spool connect the first work port with the tank port via the hollow interior of the spool. The hollow spool is center-biased by way of springs to an off-position, at which the first and second work ports are not fluidly connected with either of the pump or tank ports.
Although the hollow spool described in the technical paper may have reduced mass and, therefore, improved responsiveness, it may still be less than optimal. In particular, the integral block formed inside the hollow spool between the first and second ends moves with the hollow spool during valve actuation. As a result, the block axially displaces oil from the valve body during each movement. This displacement of oil may require a significant amount of energy, and still result in some system delay.
The disclosed valve and spool are directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.